The Deadliness of Dioxin Put in Doubt by New Data : Environment: Scientific studies may cause the federal government to raise the chemical’s level of safe exposure.

WASHINGTON —  After years of dispute, U.S. scientists finally may be closing in on an answer to one of the most controversial questions in the field of environmental pollution: Just how dangerous is dioxin?

And the response might be “bad, but not as bad as we thought.”

Ever since the 1960s, when the first studies in animals showed that dioxin could cause cancer at high doses, the chemical has been identified in popular culture with the persistent label of “most deadly carcinogen known to man.”

And, because dioxin, in at least small amounts, is produced as a byproduct of many chemical processes, that reputation has caused widespread concern and occasional massive and expensive cleanup campaigns--most spectacularly, the total evacuation and destruction of the Missouri town of Times Beach, beginning in 1982.

Moreover, as a contaminant in herbicides, such as the Agent Orange sprayed in massive amounts on Vietnam, dioxin is also the chemical blamed by many Vietnam veterans for causing illnesses from which they have suffered.

But many scientists have wondered for years whether dioxin is as serious a risk as it was generally perceived to be, and new scientific data, compiled at a conference on dioxin last year, has raised questions about many of the risk claims. Last April, acting on the results of that conference, Environmental Protection Agency chief William K. Reilly directed his agency to begin a complete review of the federal standard that governs human exposure to dioxin.

Once the study is completed, probably sometime late next year, EPA might move to dramatically increase the amount of dioxin the government considers safe, making U.S. rules more like those that have long been in effect in Canada and Europe.

That prospect makes environmentalists extremely uneasy. The EPA proposed loosening rules on dioxin in 1987, only to retreat under strong criticism from environmental groups and many scientists.

This time around, “the new information is interesting, but it’s controversial,” said Jacqueline Warren, a senior attorney at the Natural Resources Defense Council. In the past, regulators faced with uncertainty “resolved scientific questions on the side of safety.”

“Until we know, we shouldn’t take steps that will allow exposures to increase.”

At the same time, if dioxin standards are changed, many industry groups are likely to demand that chemicals they produce be re-evaluated as well, a goal that has also been pushed by the White House Office of Management and Budget.

The demand will be strongest for chemicals that are the most similar to dioxin--PCBs, a chemical formerly used as an insulator in electrical transformers that has cost billions of dollars to clean up from sites across the country.

“EPA is going to be hit with a lot of claims,” Michael Gough, an expert on dioxin at the congressional Office of Technology Assessment, predicted.

Although both the scientific and political problems of regulating dioxin are extraordinarily complicated, the underlying issue is really fairly simple. Dioxin is a deadly chemical. Give enough of it to an animal, and it will cause cancer.

But how much is enough to cause cancer--or other injuries--to humans?

Because people cannot be put in a laboratory and force-fed poisons, scientists have tried to answer such questions by using animal studies to make best guesses about safe levels of potentially unsafe chemicals. Generally, those guesses have followed what scientists call a “linear dose-response model,” meaning roughly that, if exposure to one gram of a chemical carries a 1-in-10 chance of causing cancer, exposure to half a gram would cause half that risk, exposure to a quarter gram would cause a quarter of the risk and so on.

Increasingly, however, biologists have come to doubt that the linear model accurately portrays how chemicals work in the body. In the case of dioxin, the new data seems to indicate that very low doses of the chemical have little--perhaps no--effect. The effect stays low until a threshold is reached, then increases rapidly.

Under that sort of model, “you get down to zero (risk) much more quickly,” Gough says. And, if that is true, exposures to small amounts of dioxin--and almost everyone in the United States is exposed to some small amount of dioxin, which occurs naturally--could be of much less concern.

The new studies come in two groups. The first involves studies by epidemiologists of groups of people exposed to dioxin. In the largest such study, a group led by Marilyn Fingerhut of the National Institute for Occupational Safety and Health looked at 5,172 men exposed to high levels of dioxin at 12 different industrial plants from 1942 to 1984--essentially all the plants in the United States at which workers were subjected to large amounts of the chemical.

That study, published in January in The New England Journal of Medicine, showed that men exposed to levels of dioxin up to 90 times as high as the level that the general population receives had no increased cancer risk. But men exposed to higher levels of dioxin--levels up to 500 times that which the public receives--had a 50% increased risk of cancer.

The second type of study has examined exactly how it is that dioxin works in the body. Many biologists now believe that the chemical works only indirectly, binding itself to specific proteins in animal cells that biologists call receptor sites. Once a certain number of those receptors are occupied, the combination of dioxin and the cell proteins cause changes within the cell which then, apparently, can cause disease. But, until the right number of receptors are bound up with dioxin, the changes in the cell do not seem to occur.

Even if that model is accurate, however, biologists still do not know where dioxin’s safety threshold occurs.

BACKGROUND

Dioxin is a ubiquitous pollutant produced as a byproduct in many chemical reactions, particularly those used to produce certain herbicides such as Agent Orange, a defoliant used in Vietnam. The chemical, often found as a contaminant, has no uses of its own.